Date from age given on marriage licence at the time of her marriage to Henry Nelson. Lots of Foxes in Sandwich, but no likely Mary (in index on FMP, viewed 11 Feb 2019). Too many Mary Foxes generally to have an idea who she was.

From PR entry (image on FMP, viewed 7 Feb 2019)
"Henry Nelson of Sandwch. and Mary Fox married the 6th. of June"
The baptism of a son in St. Peter's Church in 1688 does not indicate a move - the PR entry shows that they were of the parish of St. Mary's.